Non-woven fabrics are structures consisting of accumulations of fibers typically in the form of a web. Such fabrics have found great use in disposable items, such as hand towels, table napkins, curtains, hospital caps, draperies, etc., because they are far less expensive to make than conventional textile fabrics made by weaving and knitting processes.
There exist many different processes for forming non-woven structures. The processes, however, when used to generate fiber structures from fibrous stock, generally involve introducing the individualized fibers into an air stream, such that the fibers are conveyed at high velocity and high dilution rates to a condensing screen. The individualized fibers may be generated by using a lickerin or wire-wound roll to grind or shred fibrous material. There are also other techniques for generating individual fibers, e.g. through the use of various mills. The air stream is tangentially passed over the fiber-laden lickerin, or about the mill, to doff or remove the fibers and entrain them in the air stream. Typically the air stream with the fibers is contained within a duct from the point of grinding to the point of deposition upon the condenser screen. In order to maintain the air stream in the duct at velocities high enough to ensure a uniform flow and deposition of the fibers upon the condensing screen, as well as to assure that the fibers do not adhere to the duct walls, it is necessary to employ a fan or other suction device beneath the condensing screen to create a pressure of at least 20 inches of water, and often up to 100 inches of water.
U.S. Pat. No. 3,512,218 of Langdon discloses apparatus for forming non-woven webs with two lickerins. The fibers are doffed from the lickerins by a single air stream formed by a suction box below the condensing screen. U.S. Pat. No. 3,535,187 of Wood discloses a similar arrangement wherein two air streams are used to doff the fibers from the lickerin. According to U.S. Pat. No. 3,772,739 of Lovgren both pulp fibers and longer textile fibers are individualized and blended in apparatus using high speed lickerins rotating at different speeds. As in the other references, the individualized fibers are doffed from their respective lickerins by separate air streams produced by a suction fan located in the condenser section of the apparatus. A baffle plate inserted between two lickerins for controlling the degree of mixing of fibers doffed by air streams passing over separate lickerins is described in U.S. Pat. Nos. 3,768,118 of Ruffo et al. and 3,740,797 of Farrington.
In these references, and generally in the prior art, the high speed air streams impel the fibers against the condenser screen at such a speed that there is a compression of the resulting web. In addition, the particles, after leaving the lickerin, are conducted to the condensing screen by a duct structure which confines their travel and, due to the air pressure, accelerates their travel. In order to assure that the air pressure is not reduced, seal means are provided where the duct structure engages the moving condenser screen. This may be in the form of floating or rolling seals, which further act to compress the fiber web as it is withdrawn from the condenser on the moving screen.
Because of the substantial pressure which must be generated in order to create the high speed air streams, the prior art methods of producing webs require a great deal of energy. In addition, the resulting web is compressed both by the air stream and the seals that are used to maintain the pressure for the air stream. Thus, it would clearly be advantageous to the production of fluff fiber structures if they could be created with much less energy and with less compression, i.e., much greater loft.